The present invention relates to an X-ray diagnostic apparatus and radiation diagnostic apparatus for sensing an X-ray image (radiation image) of a subject.
Most of X-ray diagnostic apparatuses have an automatic exposure control function of automatically controlling the photographing time in order to keep the image density constant. The automatic exposure control is based on the following principle. An X-ray sensor is interposed between a subject and X-ray detector. An output from the X-ray sensor is temporally integrated by an integrator. The integrated value corresponds to an image density. The integrated value is compared with a reference value. When the integrated value reaches the reference value, irradiation of X-rays on the subject stops. Instead of the X-ray sensor, part of output light from an image intensifier is drawn in a photomultiplier to monitor the exposure time based on the output, or the exposure time is monitored based on part of an image signal photographed by a TV camera. Also, exposure is controlled based on an output from a planar detector prepared by arraying a plurality of semiconductor elements (Jpn. Pat. Appln. KOKAI Publication No. H7-201490). An X-ray diagnostic apparatus of this type can more appropriately control exposure, does not require any sensor other than the planar detector, and can realize cost reduction and high reliability.
In recent years, there is proposed a technique of forming a 1-frame image using not the entire detectable area of the planer detector but an arbitrary partial region (imaging region) in the detectable area (Japanese Patent Application No. H9-164249).
Since the imaging region can be arbitrarily set, as described above, the X-ray sensor for exposure-controlling may deviate from the imaging region to generate an exposure control error.
If the imaging region is set at particularly the periphery of the detectable area, X-rays are obliquely incident on the imaging region to distort an image.